Ultrasonic inspection techniques are used in many applications where non-destructive evaluation of a workpiece is required. One application of such ultrasonic inspection is in the inspection of composite fiber reinforced aircraft propeller blades. Such blades are typically formed from a plurality of layers of composite fibers (graphite, boron or S-glass, for example) laid over each other and adhesively bonded. Any separation of the fiber layers due to an incomplete bond or void in the blade may detrimentally affect blade strength. Ultrasonic inspection techniques can be used to identify and locate such flaws in a composite fiber reinforced blade.
One disadvantage of ultrasonic inspection is that ultrasound is attenuated by the material which is being inspected. Any variation in the geometry of a part being inspected will cause variation in the amplitude of any through transmission. Since anomalies in a part being inspected are detected by such amplitude variations, it becomes difficult to distinguish between actual anomalies and thickness variations.
It has been proposed to compensate for thickness or part geometry variations during ultrasonic inspection by detecting signal reflections from within the part, preferably reflections from a surface of the part opposite an ultrasound transponder. This technique is sometimes referred to as bottom echo detection. By measuring the time between transmission and reception of an ultrasound signal, and knowing the attenuation characteristics of the part being inspected, one can compute the part thickness. This technique, however, is fraught with disadvantages since internal reflections are also generated from anomalies in the part. Furthermore, relatively thick parts may require high energy ultrasound since the sound must travel through twice the thickness of the part.
With parts of varying thickness, another problem that occurs is that a change in thickness creates a change in the amplitude of a received ultrasonic signal, which change may appear as an anomaly, but which also may create a received signal amplitude which falls outside the dynamic range of the receiver. If, for example, the part varies from a thick to a thin geometry, the received signal may be so large as to saturate the receiver thus precluding obtaining of useful data.